This application is entitle to the benefit of and incorporates by reference essential subject matter disclose in Japanese Patent Application No. 2001-112683 filed Apr. 11, 2001 and Japanese Patent Application No. 2001-112684 filed Apr. 11, 2001.
The present invention relates to a sensing technique of a motor-driven power steering apparatus for a motor vehicle, and more particularly to a technique effectively applied to a motor-driven power steering apparatus that can drive a brushless motor for a motor-driven power steering at a timing corresponding to an optimum lead angle in accordance with a load.
For example, a motor-driven power steering apparatus of a technique which the inventors of the present application have studied is mounted as a steering unit for steering running wheels such as front wheels or the like of a motor vehicle, and has employed a brushless motor by considering no change in a performance thereof even if a manner of coming in contact with a commutator of a brush is changed, long service life time, and the like. As a motor-driven power steering apparatus employing the brushless motor, there has been known a so-called rack assist type structure in which the brushless motor is coaxially provided at a rack shaft and a steering assist force is obtained by the brushless motor provided at the rack shaft.
The rack assist type motor-driven power steering apparatus comprised a brushless motor, a rack shaft coaxially provided at the brushless motor, running wheels rack-and-pinion connected to the rack shaft via a connecting mechanism, a steering handle and the like, and transmits a steering assist force generated by the brushless motor, to the rack shaft via a ball-screw mechanism. Further, the structure is made such that the running wheels are steering-wheel turned by the steering assist force generated by the brushless motor and a manual steering force generated by operation of the steering handle, and thereby it is possible to reduce a steering load of a driver (for example, Japanese Patent Laid-open No. 8-251975).
However, in the motor-driven power steering apparatus using the above-mentioned brushless motor, the brushless motor is commutated and controlled in accordance with a position detecting information of a rotor. However, since a torque ripple of the brushless motor becomes sometimes large and is transmitted to the driver via a handle shaft as the play of the handle, there is the case where a bad influence is given to a steering feeling of the driver.
Further, when commutation and control thereof is executed in accordance with the position information of the rotor, the fluctuation of a power source current (electric current ripple) becomes large, for example, as shown in FIG. 7. Due to the electric current ripple, noises are generated from the brushless motor. In particular, when the rotation number of the brushless motor is high similarly to the case of quickly turning the steering handle, the noises become significant, and thereby a sense of discomfort is given to the driver.
Accordingly, an object of the present invention is to provide a motor-driven power steering apparatus which can prevent play of a handle at the time of steering and an occurrence of noises at the time of a high rotation relative to a motor-driven power steering, by taking into consideration the torque ripple of the brushless motor mentioned above and the electric current ripple of the power source current.
The present invention, in order to achieve the object mentioned above, comprises a brushless motor for generating a steering assist force; a driving means for applying one of an electric current and no electric current to each phase coil of said brushless motor and thereby making said brushless motor rotate in a predetermined direction; a load detecting means for detecting a load applied to said brushless motor; a lead-angle-amount computing means for computing a lead angle amount in accordance with the load detected by said load detecting means; a position detecting means for detecting a position of a rotor in said brushless motor; a rotation-number computing means for computing the rotation number of said brushless motor on the basis of a detection signal of said position detecting means; a lead-angle-amount correcting means for correcting the lead angle amount computed by said lead-angle-amount computing means in accordance with the rotation number computed by said rotation-number computing means; and a control means for generating a timing signal corresponding to the lead angle amount corrected by said lead-angle-amount correcting means, controlling said driving means on the basis of the timing signal, and controlling time of one of an electric current and no electric current applied to each phase coil of said brushless motor at a commutation timing corresponding to a predetermined lead angle.
Further, another object of the present invention is to provide a motor-driven power steering apparatus which can prevent an occurrence of noises at the time of steering relative to a motor-driven power steering, by taking into consideration the electric current ripple of the power source current mentioned above.
Another aspect of the present invention, in order to achieve another object mentioned above, comprises: a brushless motor generating a steering assist force; a driving means for applying one of an electric current and no electric current to each phase coil of said brushless motor and thereby making said brushless motor rotate in a predetermined direction; a position detecting means for detecting a position of a rotor in said brushless motor; a rotation-number computing means for computing the rotation number of said brushless motor on the basis of a detection signal of said position detecting means; a lead-angle-amount computing means for computing a lead angle amount in accordance with the rotation number computed by said rotation-number computing means; and a control means for generating a timing signal corresponding to the lead angle amount computed by said lead-angle-amount computing means, controlling said driving means on the basis of the timing signal and controlling time of an electric current and no electric current applied to each phase coil of said brushless motor at a commutation timing corresponding to a predetermined lead angle.